Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle-wheel sets per vehicle with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (i.e., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (i.e., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way.
Locomotives used for heavy haul applications typically must produce high tractive efforts. Good adhesion between each wheel and the surface is required for efficient operation of the locomotive. The ability to produce these high tractive efforts depends on the available adhesion between the wheel and rail. Many rail conditions such as being wet or covered with snow or ice require an application of friction enhancing agent such as sand to improve the adhesion of the wheel to the rail. Therefore, locomotives typically have sand boxes on either end of the locomotives, and nozzles to dispense the sand (both manually and automatically) to the rail on either side of-the truck.
Maximum tractive or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is slightly higher (motoring) than the true vehicle speed, i.e., the linear speed at which the vehicle is traveling, usually referred to as “ground speed” or “track speed”. The difference between tractive wheel speed and track speed is referred to as “creepage” or “creep speed.” There is a variable value of creepage at which peak tractive effort is realized. This value, commonly known as the optimal creep setpoint is a variable that depends on track speed and rail conditions. So long as the allowable creepage is not exceeded, this controlled wheel slip is normal and the vehicle will operate in a stable microslip or creeping mode. If wheel-to-rail adhesion tends to be reduced or lost, some or all of the tractive wheels may slip excessively, i.e., the actual creep speed may be greater than the maximum creep speed. Such a gross wheel slip condition, which is characterized in the motoring mode by one or more spinning axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive effort.
The peak tractive effort (TE) limits the pulling/braking capability of the locomotive. This peak tractive effort is a function of various parameters, such as weight of the locomotive per axle, wheel rail material and geometry, and contaminants like snow, water, grease, insects and rust. Contaminants in the wheel/rail interface reduce the maximum adhesion available, even at the optimal creep setpoint.
While the locomotives most often require friction enhancing agents, locomotives also require, in some situations, the application of a lubricant to reduce the wear of the locomotive wheel flanges. For example, when a locomotive is traversing a section of track with a curve. For a locomotive or a consist of locomotives that are always oriented in the same way, maximum benefit for wheel-rail wear of both the cars and the locomotives is provided by lubricating the gage side of the rail or wheel flanges on the high rail in the front and simultaneously lubricating the top of the two rails in the trailing end of the locomotive or the locomotive consist. Control of the rail gage side (RAGS) lubricator as well as the top of rail (TOR) lubricator can be done by the same controller for one locomotive or two controllers located in different locomotives for the case of a locomotive consist.
While locomotives often require increased cohesion, generally non-locomotive railway cars trailing the locomotives operate most efficiently at lower cohesion or friction levels. As such, friction modifiers such as lubricants may be added to a rail to reduce the friction and therefore pull weight of railway cars. Lubricant applied to the top of the rail and possibly to the gage side of the rail behind the last axle of the last locomotive results in reduced friction and wear of the trailing car wheels. In other systems, such as a flange lubrication system, grease is applied to the flanges of the locomotive wheels in order to reduce friction between the flange and the wheel thereby reducing fuel usage and increase rail and wheel life. The system dispenses a controlled amount of lubrication, based on locomotive speed and direction, to the inside flange of wheel to lubricate the wheel/flange interface on the trailing axles of the locomotive/train. Presently, nozzle placement is based on customer choice, and the nozzles can be applied to multiple axles and always in pairs (left and right side). The lubrication is typically of a graphite base.
It is desirable to reduce the coefficient of friction for the trailing cars as the reductions in the coefficient of friction directly reduces the pull weight and directly improves the fuel efficiency of the locomotive consist. Managing the coefficient of friction of the cars can result in a 10 to 30 percent increase in fuel efficiency.
FIG. 1 illustrates a typical prior art locomotive 122 having a friction modifying agent to increase the coefficient of friction. In this case the friction modifying agent is sand and the sanding system applies sand to the rails. Sand is stored in a short hood sand box 118 or a long hood sand box 120. The illustrated example includes eight sand nozzles 102-116. In the illustrated example, the locomotive 122 has two trucks 124 and 126; the front truck 124 has one nozzle in the front left 102, one nozzle in the front right 104, one nozzle in the rear left 106, and one nozzle in the rear right 108. The rear truck similarly has one nozzle in the front left 110, one nozzle in the front right 112, one nozzle in the rear left 114, and one nozzle in the rear right 116. Chart 128 of FIG. 1 illustrates when each of the nozzles are active. For example, sand nozzle 114 is active in the reverse direction if lead axle sand or auto sand or trainline sand is enabled.
FIG. 2 illustrates a prior art schematic diagram of the sanding system 200 of FIG. 1. The system 200 includes a compressed air reservoir 202, one sand box for each truck 204 for the front and 206 for the rear, one manual air valve for each truck (208 for the front truck and 210 for the rear truck), two electrically controlled sand valves for each truck (212 and 214 for the front truck and 216 and 218 for the rear truck), and two nozzles for each of these electrically controlled sand valves (102 and 104 for the forward front truck valves, 106 and 108 for the reverse front truck valves, 110 and 112 for the forward rear truck valves, 114 and 116 for the reverse rear truck valves). A locomotive control system 220 enables the appropriate sand valves based on the inputs from the operator or train lines, or when an adhesion control system determines that the rail conditions are poor and sanding will yield a higher tractive effort. Lubricants may be applied to the top of the rail or to the rail gage side in a similar manner (not illustrated).
FIG. 3 illustrates an exemplary adhesion creep curve 300 for a locomotive traversing a rail. As illustrated, curve 302 depicts the adhesion characteristics of dry sand that provides the highest level of adhesion for each level of per unit creep especially at per unit creep levels of less than 0.2. For per unit of creep levels of less than 0.05, wet sand as depicted by curve 304 provides a higher adhesion than a dry rail as shown by curve 306. However, at per unit creep levels greater than 0.05, wet sand curve 304 has less adhesion than the dry rail curve 306. For the situations where less adhesion is desirable, as is the case for connected railway cars or a locomotive rounding a curve in a track, oil as depicted by curve 308 provides the least amount of adhesion for per unit creep less than 0.1. Curve 310 illustrates the adhesion characteristics of water that also provides improved reduced friction as compared to a dry rail (curve 306) for per unit creep. From chart 300, it is desirable to manage the friction between a wheel of a locomotive or a railway car and the railway rails in a manner that enhances the tractive effort of the locomotive while at the same time reducing the friction of railway cars connected to the locomotive.
Chart 400 in FIG. 4 illustrates two changes in the operating point of a wheel on a wet rail when sand is applied to the wet rail (curve 402) and when sand is removed from the rail (curve 404). For example, if sand is applied to a wet rail at point 406 on water curve 310, curve 402 illustrates that the creep decreases to point 408, a point on wet sand curve 304. Similarly, if water is applied to a rail operating at point 408 on the wet sand curve 304, the removal of the wet sand moves the creep from point 408 to point 406 on curve 310, thereby indicating a significant increase in creep. FIG. 4 also illustrates optimal adhesion control system performance--creep is controlled such that maximum tractive effort is attained (assuming that the operator is calling for more tractive effort than what can be sustained by the rail conditions). Therefore, such a change can be observed by the adhesion control system only when the available adhesion at the wheel is utilized by the wheel and it typically happens at high tractive effort, low speed operating conditions. At other operating conditions the tractive effort versus creep characteristics change but not as dramatically.
In this illustration, a locomotive is applying 17,000 pounds of tractive effort. However, at point 406 the rail is wet and the wheels are experiencing a per unit creep of more than 0.14. Sand is applied immediately prior to the advancing wheel of the locomotive. As a result, at point 408 tractive effort is increased to 20,000 pounds and per unit creep is reduced to less than 0.03. If the sand is later removed, the operating point returns from point 408 to the prior operating point 406. This illustrates the benefits of both applying a friction enhancing agent, in this case sand, and the subsequent removal of the sand to thereafter reduce the friction experienced by a trailing railway car.
FIG. 5 illustrates the tractive effort in pounds as a function of the speed of the train for eight setting tractive effort or throttle settings as denoted TE1 to TE8. As shown, for a low speed there is a significant variation in the tractive effort for each of the throttle settings. However, as speed increases, the tractive effort reduces and approaches a relatively close level as the speed exceeds 50 miles per hour. It should also be noted that for each throttle setting, the tractive effort remains constant until a break speed is reached, as denoted in FIG. 5 where each line for each tractive effort drops from the level amount to a significantly lower and decreasing amount.